Hot melt adhesives are of importance because of their speed of bonding compared with other types of adhesives. No solvents must evaporate, no curing must take place, and strong adhesive bonds are obtained as soon as the adhesive cools. Polyesters have been found useful in hot melt adhesives. Many polyesters, however, have low adhesive strength, particularly peel strength, on various uncoated substrates such as aluminum, copper, steel, glass, plastic, woods, etc., and on substrates coated with various materials, such as polyepoxide resins, vinyls, phenolics, alkyds, acrylics, etc. Another problem often encountered is that some hot melt adhesives lack the ability to maintain high adhesive strength after aging as well as the ability to maintain high bond strength under relatively high temperature conditions. Moreover, many hot melt adhesives, particularly crystalline high melting polyesters, have high melt viscosities, resulting in inferior flow and wetting properties and, therefore, must be applied at relatively high temperatures.
In U.S. Pat. No. 3,657,389, hot melt adhesive blends of poly(tetramethylene terephthalate) and polystyrene are disclosed. The adhesion to certain metals of the polyester of particular inherent viscosity (I.V.) is increased by the addition of polystyrene. A similar I.V. poly(tetramethylene terephthalate) modified with 15-20 mole percent dimer acid has low adhesion on metals, but an increase in adhesion, especially to uncoated steel, occurs when the dimer acid-modified polyester is blended with polystyrene.
U.S. Pat. No. 4,094,721 relates to copolyester adhesives derived from terephthalic acid, 1,4-butanediol and 1,6-hexanediol.
U.S. Pat. No. 3,728,212 describes blends of poly(1,4-butylene terephthalate) and selected vinyl polymers as adhesives with improved bond strength. U.S. Pat. No. 3,795,644 describes blends of copolyesters of terephthalic acid, dimer acid and 1,4-butanediol with selected vinyl aromatic polymers as adhesives with increased bond strength. U.S. Pat. No. 3,915,913 describes blends of terephthalate-based polyesters with selected vinyl aromatic polymers as hot-melt adhesives. Because plastic-grade polystyrene is used in these blends, the melt viscosities of the blends are similar to that of the unmodified polyesters.